The odd object was discovered in 2007 by a Dutch schoolteacher participating in Galaxy Zoo—"voorwerp" is Dutch for "object." The online project had public citizens worldwide helping to classify galaxies in the Sloan Digital Sky Survey catalog.

Although it's the size of our Milky Way galaxy, Hanny's Voorwerp is not itself a galaxy, but a cloud of gas near the spiral galaxy IC 2497, which lies about 650 million light-years from Earth.

The new Hubble picture—the sharpest yet taken of the brilliant cloud—shows that gas in a small region of Hanny's Voorwerp is collapsing and forming stars, the youngest of which are just a couple million years old.

"The star clusters are localized, confined to an area that is over a few thousand light-years wide," study leader William Keel, of the University of Alabama in Tuscaloosa, said in a press release.

"The region may have been churning out stars for several million years. They are so dim that they have previously been lost in the brilliant light of the surrounding gas."

Previous studies looking at radio emissions from Hanny's Voorwerp showed that the blob is part of a rope of gas 300,000 light-years long, which surrounds IC 2497.

Known as a tidal tail, this rope likely formed when the spiral galaxy merged with another galaxy about a billion years ago, Keel said.

Hanny's Voorwerp is the only part of the tail that glows in visible light, because the green cloud was recently doused in radiation from the supermassive black hole at the heart of IC 2497. (Related: "Black Hole Blasts Superheated Early Universe.")

According to Keel, the galactic merger injected a glut of stars and gas into the galaxy's center, spurring the black hole to go on a feeding frenzy. This caused the black hole to begin spewing jets of radiation from its poles.

The active black hole, known as a quasar, not only set Hanny's Voorwerp aflame, it also sent out a streamer of gas that's now interacting with the green blob, triggering star formation.

Astronomers only recently solved the puzzle using x-ray observations, because the quasar is now inactive.

"We just missed catching the quasar, because it turned off no more than 200,000 years ago," Keel said. "So what we're seeing [in Hanny's Voorwerp] is the afterglow from the quasar."

Keel discussed the new Hubble Space Telescope picture of Hanny's Voorwerp today at a press briefing during the 217th meeting of the American Astronomical Society in Seattle, Washington.